Micromachining with short-pulsed, solid-state UV laser

Abstract

In some embodiments, laser output including at least one laser pulse having a wavelength shorter than 400 microns and having a pulsewidth shorter than 1,000 picoseconds permits the number of pulses used to clean a bottom surface of a via or the surface of a solder pad to increase process throughput. An oscillator module in cooperation with an amplification module may be used to generate the laser output.

1. A method for increasing laser via drilling throughput by minimizing a total number of pulses used to drill a via having desirable operational characteristics, wherein the total number N of pulses includes a bulk number N 0  of bulk removal pulses employed to remove bulk material to form the via with a laser removal-bulk material interaction and a bottom surface cleaning number δN of bottom surface cleaning pulses employed to clean a bottom surface of the via with a laser cleaning-material interaction, comprising:
 generating, from a laser, laser output having a wavelength shorter than 400 nanometers, a fluence F, and at least one laser pulse having a pulsewidth τ, shorter than 1000 picoseconds, for cleaning the bottom surface of the via, where δN has a relationship to F/τ 1/2  and such that δN/N 0  is less than or equal to 1; and 
 directing the laser output from the laser to a target position to remove a major portion of the bulk material and clean the bottom surface of a via, such that laser pulses of the laser output for removing the major portion of the bulk material and for cleaning the bottom surface of the via have a wavelength shorter than 400 nanometers and a pulsewidth shorter than 1000 picoseconds. 
 
     
     
       2. The method of  claim 1  in which the relationship between δN and F/τ 1/2  satisfies the equation: 
       
         
           
             
               
                 
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                     ( 
                     
                       
                         
                           
                             T 
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       in which L=(1/fτ) 1/2 −(1/fτ−1) 1/2 , and f is the repetition rate. 
     
     
       3. The method of  claim 1  in which δN has a relationship to L, where L=(1/fτ) 1/2 −(1/fτ−1) 1/2  and f is the repetition rate. 
     
     
       4. The method of  claim 1  in which multiple vias are formed and cleaned in a single pass, such that the laser is directed by a beam positioning system to address each target position once to perform both operations of bulk material removal and cleaning. 
     
     
       5. The method of  claim 1  in which the laser output comprises a wavelength of about 355 nanometers or 351 nanometers. 
     
     
       6. The method of  claim 1  in which the laser output has a repetition rate between 10 megahertz and 100 megahertz. 
     
     
       7. The method of  claim 1  in which the laser pulse has a pulsewidth that is shorter than 500 picoseconds. 
     
     
       8. The method of  claim 1 , further comprising: employing image-shaping optics to shape the laser output. 
     
     
       9. The method of  claim 1  in which the via is drilled in a printed wiring board. 
     
     
       10. The method of  claim 1  in which the bulk material includes a homogenous film, a particulate-filled resin, a polyimide, or a fiber-reinforced polymer. 
     
     
       11. The method of  claim 10  in which the bulk material includes a metal cladding. 
     
     
       12. The method of  claim 11  in which the metal cladding is less than about 18 microns thick. 
     
     
       13. The method of  claim 1  in which the bottom surface material includes a metal. 
     
     
       14. The method of  claim 1  in which the laser output is generated by a solid-state laser or a fiber laser. 
     
     
       15. The method of  claim 1  in which the laser output is employed in a laser punching process. 
     
     
       16. The method of  claim 1  in which the bulk material includes a fiber-reinforced polymer and the via has sidewalls exhibiting minimal fiber protrusion. 
     
     
       17. The method of  claim 1  in which the laser output has a repetition rate that is less than or equal to 10 megahertz. 
     
     
       18. The method of  claim 1  in which the laser output has a repetition rate that is less than or equal to 1 megahertz. 
     
     
       19. The method of  claim 1  in which the bulk material removal and bottom surface cleaning are performed at about the same fluence but at different pulsewidths. 
     
     
       20. The method of  claim 1  in which bottom surface cleaning removes less than a 2-micron thickness of material. 
     
     
       21. The method of  claim 1  in which the via drilling is accomplished by a punching process. 
     
     
       22. The method of  claim 1  in which the via drilling is accomplished by a trepanning, spiraling, or looping process. 
     
     
       23. The method of  claim 1  in which a workpiece at the target position includes an overlying metal layer, a bulk material positioned beneath the overlying metal layer, and an underlying metallic layer or pad positioned beneath the bulk material, and in which the top metal layer is removed with a first set of laser parameters, the bulk material is removed with a second set of laser parameters, and the underlying metallic layer is cleaned with a third set of laser parameters, wherein the first, second, and third sets of parameters are different. 
     
     
       24. A method for laser drilling a blind via in a workpiece including a bulk material and an underlying bottom surface material wherein a laser-bulk material interaction determines a bulk set of optimal laser processing parameters for efficiently removing a major portion of the bulk material to form the blind via and wherein a laser cleaning-material interaction determines a cleaning set of optimal laser processing parameters for efficiently cleaning the bottom surface with a fewest number of bottom surface-cleaning pulses employed to clean the bottom surface of the blind via without causing operational damage to the bottom surface of the blind via, such that the cleaning set of optimal laser processing parameters is less efficient for removing bulk material than is the bulk set of optimal laser processing parameters, comprising:
 determining for a wavelength shorter than 400 nm optimal ranges of fluence, pulsewidth, and repetition rate within the cleaning set for minimizing the number of laser pulses to clean the bottom of a blind via, 
 employing a solid-state laser to generate laser output at a wavelength shorter than 400 nanometers having parameters within the cleaning set of optimal laser processing parameters, the laser output including multiple laser pulses having pulsewidths shorter than 500 picoseconds; 
 employing image-shaping optics to shape the laser output; and 
 directing the multiple laser pulses of the laser output at a target position to remove the major portion of the bulk material to form the blind via in the workpiece and to clean the bottom surface material. 
 
     
     
       25. The method of  claim 24  in which the bulk material includes a homogenous film, a particulate filled resin, a polyimide, or a fiber-reinforced polymer. 
     
     
       26. The method of  claim 25  in which the bulk material includes a metal cladding. 
     
     
       27. The method of  claim 26  in which the metal cladding is less than about 18 microns thick. 
     
     
       28. The method of  claim 24  in which the bottom surface material includes a metal. 
     
     
       29. The method of  claim 24  in which the bulk material includes a resistive material. 
     
     
       30. The method of  claim 24  in which the laser output has a repetition rate that is less than or equal to 10 megahertz. 
     
     
       31. The method of  claim 24  in which multiple vias are formed and cleaned in a single pass, such that the laser is directed by a beam positioning system to address each target position once to perform both operations of bulk material removal and cleaning. 
     
     
       32. The method of  claim 24  in which the laser output has a repetition rate between 10 megahertz and 100 megahertz.